A natriuretic peptide family consists of at least three types of natriuretic peptides, atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C type natriuretic peptide (CNP). CNP is a vascular proliferation-regulating peptide mainly secreted from endotherial cells. ANP and BNP are cardiac hormones mainly synthesized in and secreted from heart. These peptides are synthesized as pro-hormones and cleaved to be mature peptides, α-ANP, α-BNP, α-CNP respectively. Human α-ANP, α-BNP, and α-CNP consist of 28, 32, and 22 amino acid residues, respectively.
Some diseases cause the secretion of these natriuretic peptides into blood stream. Since the synthesis and secretion of ANP and BNP are promoted mainly by a load against atria and ventricles of heart, respectively, their secretions reflect changes of heart functions. Each peptide is used as a diagnostic indicator of heart diseases, especially heart failure. Measurement of both α-ANP and α-BNP by immunoassay has already been applied in the clinical field.
Since α-ANP and α-BNP are easily degraded by proteases in blood after the collection, they are extremely unstable in blood samples. Thus results of measurement had been greatly affected by the collecting methods, storing methods of specimens and the period from collection to measurement. To measure the concentration of the peptides exactly, addition of degradation inhibiting agents, e.g., aprotinin etc. or keeping specimens at low temperature had been essential. But, these handlings were complicate, required too many tasks, and not completed methods as pretreatment of specimens.